The present invention relates to a controlling technology for an electronic camera called a digital camera, and in particular, to a technology capable of making a whole image to be in an appropriate luminance.
There has recently been put to practical use an electronic camera which is of the structure wherein an optical image of a subject is focused on a solid image sensor (CCD) through an optical photographing system such as a photographing lens and an iris to be converted photoelectrically and outputted by the image sensor as electric image signals which are recorded on a recording medium.
In an ordinary automatic exposure control (hereinafter referred to as AE) of an electronic camera of the above-mentioned type, an iris and exposure time (hereinafter referred collectively to as exposure amount) are controlled so that a value obtained by integrating and averaging luminance signal levels of the whole image detected by the image sensor may be at an appropriate level.
In addition to that, there is also another one wherein luminance signal levels are integrated by making the weighting on the image central portion greater, and the value obtained from the integration is used to control an exposure amount.
However, in the ordinary method to use the value obtained by integrating and averaging luminance signal levels on the whole photographed image, when there is a portion of extremely high luminance on a part of an image, for example, the integrated value is influenced by that portion to be enhanced. Therefore, if the exposure amount is determined by using that average value as a standard, the image results in a dark one totally and a signal-to-noise ratio is also deteriorated. Further, a high luminance portion tends to be too luminous, resulting in a white washout image, because an exposure amount is determined to be matched to an average value of a high luminance portion and a low luminance portion.
In the case of the exposure control wherein an image central portion is weighted, too, when there is a portion of extremely high luminance in a highly weighted area, the problem mentioned above can not be solved.
Though there is further a method wherein an exposure amount is corrected by photographing several times and thereby the exposure is controlled so that a primary subject is in an appropriate luminance, it is sometimes impossible to make a portion other than the primary subject to be in an appropriate luminance, and in particular, it has been impossible for the exposure control alone to solve the aforesaid problem that a high luminance portion turns to be too luminous, resulting in a white washout image.
The invention has been achieved by paying an attention to the above-mentioned problems in the past, and an object of the invention is to make a primary subject to be in an appropriate luminance, to establish an optimum signal-to-noise ratio, to prevent that a high luminance portion turns to a white washout image, and thereby to obtain images with high image quality, in an electronic camera.
The above mentioned object is achieved by the camera and a method of controlling the camera.
The method includes the steps of: obtaining image signals from an image sensor by having the image sensor detect an image under a predetermined exposure amount; detecting luminance distribution condition of the image signals; and determining another exposure amount and a gradation characteristic for secondly obtaining image signals in relation to each other according to the previously detected luminance distribution condition.
The camera includes: an image sensor for converting image information to electric signals so as to obtain image data; a detector for detecting a luminance distribution of the image data; an exposure amount determination circuit for determining an exposure amount of photographing according to the luminance distribution; and a gradation characteristic determination circuit for determining a gradation characteristic of the image data in relation to the exposure amount of photographing.
Due to the arrangement stated above, by determining an exposure amount and a gradation characteristic so that they are properly correlated with each other, based on the luminance distribution condition of an image, it is naturally possible to make a primary subject to be appropriately luminous, and it is further possible to control so that a portion other than the primary subject is also made to be appropriately luminous, and a signal-to-noise ratio is also improved.
The controlling method or the apparatus for an electronic camera related to the invention makes an exposure amount determining means to determine an exposure amount to be smaller slightly when a difference of luminance in the luminance distribution condition of the image is great, and to determine the exposure amount to be larger slightly when the luminance difference is small, and it makes the gradation characteristic determining means to determine a gradation characteristic so that a high luminance portion is hardly saturated.
Due to the arrangement stated above, it is possible to reduce a high luminance portion in an image first by determining an exposure amount to be smaller slightly when a difference of luminance in the luminance distribution condition of an image is great, and it is possible to reduce frequency of occurrence of a white washout image by determining a gradation characteristic so that the high luminance portion is hardly saturated. Incidentally, making an exposure amount to be small makes a primary subject to be appropriately luminous in many cases usually without making it to be too luminous. However, when luminance of the primary subject is insufficient, it is possible to correct with a gradation characteristic so that the luminance on the primary subject may be enhanced.
On the contrary, when a difference of luminance in the luminance distribution condition of an image is small, an exposure amount is determined to be larger slightly, and thereby the luminance distribution is broadened to secure an appropriate contrast, and a signal-to-noise ratio is also improved. Even in this case, a gradation characteristic is determined so that a high luminance portion is hardly saturated to prevent occurrence of a white washout image, and when a primary subject is too luminous due to the increased exposure amount, it is possible to correct with a gradation characteristic so that the luminance of the primary subject may be lowered.
The controlling method or the apparatus for an electronic camera related to the invention is characterized in that an exposure amount determining means and a gradation characteristic determining means determine respectively an exposure amount and a gradation characteristic based on the luminance distribution condition of the image so that a luminance area with high frequency may represent the standard luminance.
Due to the arrangement stated above, it is possible to control a primary subject portion to be in appropriate luminance in many cases because the primary subject portion is usually intended to be photographed to be large in size and thereby it is highly probable that a luminance area with high frequency is a primary subject portion.
The controlling method or the apparatus for an electronic camera related to the invention is characterized in that an exposure amount determining means and a gradation characteristic determining means determine respectively an exposure amount and a gradation characteristic so that luminance on the central portion of an image may represent the standard luminance.
Due to the arrangement stated above, it is possible to control a primary subject portion to be in appropriate luminance in many cases because the primary subject portion is usually intended to be photographed to be large in size and to be located at the central portion and thereby it is highly probable that a luminance area with high frequency is a primary subject portion.
Further, the controlling method or the apparatus for an electronic camera related to the invention is characterized in that an exposure amount determining means and a gradation characteristic determining means determine respectively an exposure amount and a gradation characteristic so that average luminance of the whole image may represent the standard luminance.
Due to the arrangement stated above, it is possible to control the average luminance of the whole image appropriately.
Further, the controlling method or the apparatus for an electronic camera related to the invention is characterized in that a gradation characteristic is determined by a gradation characteristic determining means based on the luminance distribution condition of the image photographed again by the use of the exposure amount determined by the exposure amount determining means.
In the above-mentioned arrangement, it is possible to determine a gradation characteristic more accurately based on the luminance distribution condition of the image photographed actually using the exposure amount determined appropriately, and thereby to improve image quality sharply.